The dramatic decrease is the result of a coordinated attack by security firms and Internet service providers around the globe that took down a network of infected computers known as "the Grum botnet." Grum, one of the world's most prolific spammers, generated around 18 billion emails a day, by FireEye's estimates.

A botnet is a collective of computers infected with malware -- typically without the computer owner's knowledge -- and taken over by an outside attacker. Criminals who gain control of botnets use them for malicious activities like pumping out massive volumes of spam or launching denial-of-service attacks on targeted websites. The bigger the botnet, the more firepower the cybercrimal has at their fingertips.

Grum was an especially vast and nasty spammer. First detected in early 2008, its malware infected several hundred thousand computers around the world and churned out huge amounts of pharmaceutical spam advertising cheap drugs.

At its peak, Grum was the world's most prolific spam machine, though researchers recently dropped it to the number three spot on their ever-changing list of the world's largest botnets.

The tale of its demise reads like a high-tech thriller.

The brain of a botnet is what's known as a "command and control" server. Grum had several of those servers scattered around the globe in countries including Russia, Panama, and the Netherlands. But it also had a fatal weakness: The network had no recovery mechanism if all of its command servers were simultaneously knocked offline.

A Dutch Internet service provider yanked the plug Tuesday on two of Grum's primary command servers. A Panamanian server went down next, leaving just one main server -- in Russia -- coordinating the entire Grum swarm.

But when the botnet's operators realized their network was under attack, they launched their evasive actions, shifting their traffic to a fresh set of backup servers in Ukraine.

"Right in front of my eyes, the bot herders started pointing their botnet to new destinations," Mushtaq wrote in a blog post about the takedown. "For a moment, I was stunned."

Mushtaq alerted collaborators around the globe, including a cybersecurity team in Russia that quickly went after the new servers' Internet providers. Within a few hours, they persuaded key providers to cut the connection. By 2 p.m. ET on Wednesday, the entire system was dead.

"We are confident that it can't recover," Mushtaq told CNNMoney on Thursday morning. "I've been monitoring Grum for four years. Right from the start we knew that it doesn't have any fallback mechanism."

Grum was responsible for 35% of the Internet's spam volume last week, according to monitoring statistics from security firm Trustwave.

Tracking botnet spam is tricky, and other firms have different estimates. Spam tracker Spamhaus estimates that 15% to 17% of the world's spam was coming from Grum as of early this week.

Its demise is having ripple effects. The spam volume from another major botnet, Lethic, plunged overnight, Mushtaq said. He thinks the operators of that botnet have "gone underground."